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Description/Abstract

We show that in spite of common beliefs, light polarization rotation non-reciprocity may be achieved in a non-magnetic structure. Waves diffracted on a regular planar chiral array show different polarization eigenstates in the direct and reversed diffraction scenarios in non-zero diffraction order. If the diffracted light wave is reflected straight back towards the planar chiral structure by a mirror and then diffracts again, the polarization azimuth of the returning wave is different from the incident light by tenths of a degree, resembling the famous non-reciprocity of the Faraday effect. The effect is compatible with the Lorenz lemma, while both structural chirality of the array arrangement and the chirality of individual elements of the array give rise to polarization non-reciprocity.

The question of whether an optical phenomenon is reciprocal or not is answered by comparing results of two experiments in which the directions of light propagation are mutually opposite. This comparison shall establish whether eigenwaves (characterized by polarization eigenstates with corresponding propagation constants) are the same in both directions or not. Systems with identical eigenwaves in both directions are reciprocal, while systems for which they are different are nonreciprocal. For example, conventional crystal birefringence is a truly reciprocal effect because linearly polarized, mutually perpendicular eigenwaves do not depend on the light propagation direction. Similarly, circularly polarized eigenwaves in an isotropic optical active medium remain unchanged if the propagation direction is reversed, thus making 3D optical activity a true reciprocal phenomenon. In contrast, in the non-reciprocal optical Faraday effect, two circularly polarized eigenwaves swap over for opposite propagation directions. The latter leads to a non-reciprocal polarization azimuth rotation and makes possible unidirectional devices such as optical isolators. It is a common belief that non-reciprocity may only be achieved in magnetic materials which is what we wish to challenge in this paper. Recently we reported that planar (2D) chiral structures affect the polarization state of light in an enantiomeric fashion, similarly to three-dimensional chiral media [1]. However, the sign of planar chirality reverses if the structure is observed from different sides of the plane, and so should the polarization effect associated with it. Non-reciprocity due to opposite sense of rotation has never been studied theoretically before, leaving the fundamental difference between 2D and 3D chirality as yet not fully understood. One another motivation for this study was the recently observed broken time reversal evident in polarized optical images of planar chiral structures.

Here report on the results of an investigation, which reveals and explains a strong non-reciprocity of polarization change of light in non-zero diffraction order, diffracted on regular arrays of planar chiral metallic structures. By performing first principle numerical and analytical analysis of the diffraction process we found that the polarization non-reciprocity of diffraction is linked to the planar chirality of the structure, and could be induced by either chirality of the individual elements of the array, or by arranging non-chiral elements of the array in a chiral fashion